Multi-threshold smoothing

ABSTRACT

Systems and techniques are provided for using a multi-threshold buffer model to smooth data transmission to a client. A client to receive data such as streaming media data may include a buffer. A server to transmit data to the client may transmit the data to the client at a first server transmission rate. The client buffer level may be determined at different times; for example, a first buffer level may be determined at a first time and a second buffer level may be determined at a second time. If the buffer level is at a threshold level or has passed a threshold level between the first and second times, server transmission rate information may be determined and may be sent to the server.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to co-assigned U.S. Provisional Patent Application No. 60/352,071, entitled “A MULTI-THRESHOLD ONLINE SMOOTHING TECHNIQUE FOR VARIABLE RATE MULTIMEDIA STREAMS,” filed on Jan. 25, 2002, which is hereby incorporated by reference in its entirety.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

[0002] The invention described herein was made in the performance of work funded in part by NSF grants EEC-9529152 (IMSC ERC) and IIS-0082826, and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202) in which the contractor has elected to retain title.

BACKGROUND

[0003] Many multimedia applications, such as news-on-demand, distance learning, and corporate training, rely on the efficient transfer of pre-recorded or live multimedia streams between a server computer and a client computer. These media streams may be captured and displayed at a predetermined rate. For example, video streams may require a rate of 24, 29.97, 30, or 60 frames per second. Audio streams may require 44,100 or 48,000 samples per second. An important measure of quality for such multimedia communications is the precisely timed playback of the streams at the client location.

[0004] Achieving this precise playback is complicated by the popular use of variable bit rate (VBR) media stream compression. VBR encoding algorithms allocate more bits per time to complex parts of a stream and fewer bits to simple parts, in order to keep the visual and aural quality reasonably uniform. For example, an action sequence in a movie may require more bits per second than the credits that are displayed at the end.

[0005] VBR compression may result in bursty network traffic and uneven resource utilization when streaming media. Additionally, due to the different transmission rates that may occur over the length of a media stream, transmission control techniques may need to be implemented so that a client buffer neither underflows or overflows. Transmission control schemes generally fall within one of two categories: they may be server-controlled or client-controlled.

[0006] Server-controlled techniques generally pre-compute a transmission schedule for a media stream based on a substantial knowledge of its rate requirements. The variability in the stream bandwidth is smoothed by computing a transmission schedule that has a number of constant-rate segments. The segment lengths are calculated such that neither a client buffer overflow nor an underflow will occur.

[0007] Server-controlled algorithms may use one or more optimization criteria. For example, the algorithm may minimize the number of rate changes in the transmission schedule, may minimize the utilization of the client buffer, may minimize the peak rate, or may minimize the number of on-off segments in an on-off transmission model. The algorithm may require that complete or partial traffic statistics be known a-priori.

[0008] Client-controlled algorithms may be used rather than server-controlled algorithms. In a client-controlled algorithm, the client provides the server with feedback, instructing the server to increase or decrease its transmission rate in order to avoid buffer overflow or starvation.

SUMMARY

[0009] Systems and techniques are provided for using a multi-threshold buffer model to smooth data transmission to a client.

[0010] In general, in one aspect, a method includes receiving data such as streaming media data from a server transmitting the data at a first transmission rate. At least some of the received data is stored in a buffer. The buffer level is determined at different times. For example, a first buffer level is determined at a time, and a second buffer level is determined at a later time. The different buffer levels are compared to a plurality of buffer thresholds. For example, the first buffer level and the second buffer level are compared to the buffer thresholds to determine if one or more of the buffer thresholds is in the range between the first buffer level and the second buffer level (where the range includes the first buffer level and the second buffer level).

[0011] If at least one threshold is in the range, a second server transmission rate may be determined, based on the at least one threshold. The second server transmission rate may be predetermined (e.g., may be chosen from a list), or may be calculated.

[0012] Information based on the second server transmission rate may be transmitted to the server. For example, the second server transmission rate may be transmitted, or a change in server transmission rate may be transmitted. If the second server transmission rate is not different than the first transmission rate, rate information may or may not be transmitted to the server.

[0013] The second server transmission rate may be based on a difference between a buffer level and a target buffer level. Different methods may be used to determine second server transmission rates, depending on which threshold is in the range from the first buffer level to the second buffer level. For example, a first calculation method may be used to determine the second server transmission rate if a particular threshold is in the range, while a second calculation method may be used if a different threshold is in the range. Alternately, the second server rate may be calculated for a particular threshold, and may be chosen for a different threshold.

[0014] The second server transmission rate may be based on one or more predicted future consumption rates. Future consumption rates may be predicted using one or more past consumption rates. Future consumption rates may be predicted using a prediction algorithm. For example, an average consumption rate algorithm, an exponential average consumption rate algorithm, or a fuzzy exponential average algorithm may be used. One or more weighting factors may be used in the prediction algorithm.

[0015] In general, in one aspect, a method for transmitting data such as continuous media data includes transmitting data at a first transmission rate, receiving a communication from a client including rate change information, and transmitting additional continuous media data at a second transmission rate based on the rate change information. The rate change information may be determined using a plurality of buffer threshold levels.

[0016] The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

[0017]FIG. 1 is a schematic of a system for implementing multi-threshold smoothing.

[0018]FIG. 2 is a schematic of a buffer model including multiple thresholds.

[0019]FIG. 3A illustrates arithmetic threshold spacing.

[0020]FIG. 3B illustrates geometric threshold spacing.

[0021]FIG. 3C shows the normalized throughput using three threshold spacing schemes.

[0022]FIG. 4 shows process steps for implementing multi-threshold smoothing.

[0023]FIG. 5 is a schematic of a fuzzy exponential average algorithm.

[0024]FIGS. 6A and 6B illustrate membership functions for the variables var and α_(cr).

[0025]FIG. 7 shows the dependence of normalized throughput on the number of thresholds, for three buffer sizes.

[0026]FIGS. 8A to 8C show the dependence of normalized throughput on the size of the prediction window, for three buffer sizes.

[0027]FIG. 9 shows the number of rate changes for different threshold numbers and for three buffer sizes.

[0028]FIGS. 10A through 10C show the number of rate changes for different prediction window sizes, and for three buffer sizes.

[0029] Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0030] As explained above, server-controlled or client-controlled algorithms may be used for transmission control of streaming continuous media data. Server-controlled techniques may have several disadvantages. For example, they may not work with live streams where only a limited rate history is available. Additionally, they may not adjust to changing network conditions, and they may get disrupted when users invoke interactive commands such as pause, rewind, and fast forward.

[0031] Client-controlled algorithms may be a better choice in a dynamic environment. A client-controlled technique may more easily adapt to changing network conditions. In addition, a simpler and more flexible architecture may be used, since the server does not need to be aware of the content format of the stream. Therefore, new media types such as “haptic” data can automatically be supported without modification of the server software.

[0032] However, available client-controlled techniques have a number of drawbacks, including feedback overhead and response delays. Available techniques may not adapt sufficiently quickly to avoid buffer starvation or overflow.

[0033] The present application is directed to systems and techniques for providing continuous media data to end users effectively. Using multi-threshold flow control (MTFC), the current systems and techniques may avoid buffer overflow or starvation, even when used in a bursty environment such as a VBR environment. Unlike some available techniques, a-priori knowledge of the actual bit rate for the media stream is not necessary.

[0034] The current systems and techniques may be implemented in LAN, WAN, or other network environments, with a range of buffer sizes and prediction windows. Referring to FIG. 1, a system 100 includes a server 110, a network 120, and a client 130. For a continuous media system that may be used, please see the pending U.S. patent application entitled “CONTINUOUS MEDIA SYSTEM,” filed Jan. 24, 2003, attorney reference number 06666-127001, which is hereby incorporated by reference in its entirety. For information on a data placement technique that may be used with the current systems and techniques, please see the pending U.S. Patent Application entitled “PSEUDORANDOM DATA STORAGE,” filed Jan. 24, 2003, attorney reference number 06666-129001, which is hereby incorporated by reference in its entirety.

[0035] Client 130 includes a client buffer 140, with a capacity equal to B. Client buffer 140 may be used to store data prior to decoding and display/playback. For example, when client 130 is receiving streaming video data from server 110, buffer 140 stores data to be subsequently decoded by a media decoder and displayed to the user. If buffer 140 overflows, some of the data may be lost, and there may be a “hiccup” in the display. Similarly, if buffer 140 empties (i.e. “starves,”) there may be a hiccup in the display until additional data is received. Therefore, managing the buffer level is important to providing a high quality display or playback to an end user.

[0036] Client 130 (and/or associated machines) also includes circuitry and/or software for implementing multi-threshold flow control. For example, client 130 can receive streaming media data from the server (i.e., client 130 has a network connection), can store at least some of the data in buffer 140 prior to decoding using a decoder (e.g., implemented in hardware and/or software), can determine the buffer level at different times, and can determine whether one or more thresholds has been passed since a previous determination of a buffer level. Client 130 also includes circuitry and/or software to implement a prediction algorithm, and to determine a new server sending rate and/or rate change, and to transmit the server transmission information to server 110.

[0037] Similarly, server 110 (and/or one or more associated machines) includes circuits and/or software for transmitting continuous media data to one or more clients, for receiving communications from the one or more clients, and for updating a server transmission rate based on transmission information contained in a communication from the one or more clients.

[0038] Buffer Model

[0039] Referring to FIG. 2, a model 200 of buffer includes a number of watermarks 210. Watermarks 210 include an underflow protection watermark 210-U and an overflow protection watermark 210-O. Watermarks 210 include one or more intermediate watermarks W_(i) such as watermarks 210-1 to 210-N of FIG. 2.

[0040] Watermark 210-U is set at an underflow threshold protection level; that is, a percentage of the buffer capacity that indicates that buffer starvation may be imminent. Watermark 210-1 marks a low buffer warning threshold. When the buffer level falls below watermark 210-1, the buffer is nearing starvation.

[0041] Similarly, watermark 210-O is set at an overflow threshold protection level; that is, a percentage of the buffer capacity that indicates that buffer overflow may be imminent. The overflow threshold protection level may be the same as or different than the underflow threshold protection level. Watermark 210-N is the overflow buffer warning threshold.

[0042] Using a model such as buffer model 200 may allow smooth streaming of media data from the server to the client. The number of intermediate watermarks N may be varied to provide greater control over the buffer level (larger N) or to provide less control with fewer rate adjustments (smaller N). The watermark spacing may be equidistant, based on an arithmetic series (see FIG. 3A), based on a geometric series (see FIG. 3B), or may be set using a different method.

[0043]FIG. 3C shows the normalized throughput obtained using each of the three above-mentioned spacing methods. Generally, the performance of each of the spacing methods is somewhat similar, with some poor results for particular combinations of buffer size and spacing method (e.g., for an 8 MB buffer with 17 thresholds, geometric spacing underperforms both equidistant and arithmetic spacing).

[0044] For equidistant spacing, the underflow and overflow thresholds may first be determined. For example, the underflow threshold may be set as 5% of the buffer capacity, and the overflow threshold may be set as 95% of the buffer capacity.

[0045] The number of intermediate watermarks N may be chosen (e.g., selected or predetermined), with N>1. More typically, the number of intermediate watermarks is greater than one, for smoother traffic (see, e.g., FIG. 7 and the related discussion, below). Denoting the threshold for the underflow watermark as W_(u), the threshold for the overflow watermark as W_(o), the thresholds W_(i) for each of the intermediate watermarks i=1 through i=N are then given by: $\begin{matrix} {W_{i} = {W_{U} = {i \times \frac{W_{O} - W_{U}}{N + 1}}}} & {{Equation}\quad (1)} \end{matrix}$

[0046] Transmission Smoothing

[0047] Whenever a threshold W_(i) is crossed, a new server sending rate may be calculated, and a rate adjustment (or equivalently, a new server sending rate) may be sent to the server. For example, when the RTSP protocol is used, the information may be sent to the server using an RTSP feedback command. The action taken may depend on which threshold has been crossed. For example, if the W_(o) or W_(u) thresholds are crossed, more aggressive action may be taken than if one of the intermediate thresholds W_(i) is crossed. Similarly, if the warning thresholds W_(1 or W) _(N) are crossed, the action taken may be more aggressive than if a different intermediate threshold W_(i) had been crossed, but may be less aggressive than if W_(o) or W_(u) had been crossed.

[0048] For example, if the W_(o) threshold is exceeded, the server may be paused (i.e., the sending rate may be set to zero), or its sending rate substantially decreased. The server may remain paused until the buffer level crosses a particular threshold or a particular value (e.g., the N/2 threshold, or the mid-point of the buffer capacity).

[0049] Similarly, if the buffer W_(u) threshold is crossed, the server sending rate may be increased substantially; for example, it may be increased to about one and a half times the average server sending rate until the buffer level reaches a particular value or threshold. When the intermediate thresholds are crossed, new server sending information may be determined by choosing particular rate change amounts or by calculating new server sending information as described below.

[0050] In a simple system, the rate change amounts may be predetermined. For example, in an implementation with five intermediate thresholds W_(1-W) ₅, the interval between packets may be set to 20% less than a default interval for W₁, to 10% less than a default interval for W₂, to the default interval for W₃, to 10% greater than the default interval for W₄, and for 20% greater than the default interval for W₅.

[0051] Referring to FIG. 4, method steps 400 for implementing multi-threshold transmission smoothing are shown. Overflow and underflow thresholds may be chosen (410). The number and spacing of intermediate watermarks may be selected (420). The spacing may be selected according to one of the spacing schemes described above, or may be set in a different way (e.g., chosen).

[0052] In operation, the server transmits streaming media data to a client at a server transmission rate (430). The client receives the streaming media data and stores at least some of the data in a buffer prior to decoding (440). At intervals, the buffer level is determined and compared to the previous buffer level to determine whether one or more thresholds has been crossed (450). If a threshold has been crossed, server transmission information (e.g., a new server transmission rate and/or a rate change) is calculated (460), and if it is different from the previous server transmission rate, the server transmission information is communicated to the server (470). Note that the method steps of FIG. 4 need not be performed in the order given.

[0053] Techniques to provide data transmission smoothing may use a number of different components and variables. Table 1 includes a list of the parameters used herein. TABLE 1 TERM DEFINITION B Buffer size N Number of intermediate threshold levels W_(i) i-th intermediate threshold level W_(O) Overflow threshold level W_(U) Underflow threshold level W_(R) Resume threshold (e.g., B/2) b_(t) Buffer target level (e.g., B/2) Δt_(obs) Sampling interval t_(obs) (i) i-th observation point t_(ctrl) (i) i-th control point w_(obs) Observation window size; i.e., the number of observation points S Observed server transmission rate window, <s1, s2, . . . , s_(wobs)> S_(i) i-th latest observed server transmission rate S_(wobs) Latest observed server transmission rate R Observed client consumption rate window, <r₁, r₂, . . . , r_(wobs)> r_(i) i-th latest observed client consumption rate r_(wobs) Latest observed client consumption rate B_(obs) Observed client buffer status window, <b₁, b₂, . . . b_(wobs)> b_(i) i-th latest observed client buffer status b_(wobs) Latest observed client buffer status W_(pred) Prediction window size; i.e., the number of predicted points {circumflex over (R)} Predicted client consumption rate window, <{circumflex over (r)}₁, {circumflex over (r)}₂, . . . , {circumflex over (r)}_(wpred)> {circumflex over (r)}_(i) i-th predicted future client consumption rate (consumption rate at time t_(obs) (w_(obs)) + Δt_(obs) × W_(pred)) {circumflex over (r)}_(wpred) The furthest predicted future client consumption rate T_(correct) Expected duration w_(pred) × t_(obs) to recover the current buffer status to b_(t) S_(new) Computed new server transmission rate W_(fcd) Observation feedback control delay window size SCR[i] i-th latest smoothed consumption rate {circumflex over (r)} Predicted client consumption rate α_(cr) Client consumption rate predict parameter r_(t) Observed client consumption rate at time t {circumflex over (r)}_(t) Predicted client consumption rate at time t Δr Rate changes C Amount of data consumed during the prediction window d_(feedback) Feedback control delay

[0054] Rate Change Computation

[0055] In order to determine an amount by which the server sending rate may be adjusted, the server sending rate, the decoder consumption rate, and the buffer level may be sampled at time intervals equal to Δt_(obs). If the observed buffer level b_(obs) crosses any of the thresholds W_(i), a new server sending rate is computed using Equation (2A) below, and the related rate change Δr is shown in Equation (2B). $\begin{matrix} {s_{new} = \frac{\left( {b_{t} - b_{wobs} + C} \right) - \left( {s_{wobs} \times d_{feedback}} \right)}{\left( {w_{pred} \times \Delta \quad t_{obs}} \right) - d_{feedback}}} & {{Equation}\quad \left( {2A} \right)} \\ {{\Delta \quad r} = {1 - \frac{s_{new}}{s_{wobs}}}} & {{Equation}\quad \left( {2B} \right)} \end{matrix}$

[0056] Equation (3) below shows how C is related to the predicted future consumption rates {circumflex over (r)}_(i). (the prediction of future consumption rates is discussed more fully below): $\begin{matrix} {C = {\sum\limits_{i = 1}^{w_{pred}}\left( {{\hat{r}}_{i} \times \Delta \quad t_{obs}} \right)}} & {{Equation}\quad (3)} \end{matrix}$

[0057] When crossing the thresholds W₁ and W_(N), the computed rate change Δr may not be sufficient to avoid reaching W_(U) and W_(o), respectively, due to the error margin of the prediction algorithms. Although the error margin may be reduced, doing so adds computational complexity that may not be desired in certain situations.

[0058] An alternative is to add or subtract a mean absolute percentage error (MAPE) from {circumflex over (r)}_(i), as shown in Equations (4A) and (4B). Equation (4A) shows how an adjusted {circumflex over (r)}_(i) may be calculated when W_(N) is reached, while Equation (4B) shows how an adjusted {circumflex over (r)}_(i) may be calculated when W₁ is reached.

{circumflex over (r)} _(i)(adjusted)={circumflex over (r)} _(i)×(1−MAPE)   Equation (4A)

{circumflex over (r)} _(i)(adjusted)={circumflex over (r)} _(i)×(1−MAPE)   Equation (4B)

[0059] Equation (5) shows how a MAPE value may be computed. In Equation (5), P is the number of prediction samples up to the current prediction time. $\begin{matrix} {{MAPE} = {\left( \frac{1}{P} \right){\sum\limits_{t = 1}^{P}{{r_{t} - {\hat{r}}_{t}}}}}} & {{Equation}\quad (5)} \end{matrix}$

[0060] Consumption Rate Prediction

[0061] Rather than requiring knowledge of the bit rate of the media stream prior to transmission, the current systems and techniques predict a consumption rate, so that live streams (e.g., streams that are being produced and transmitted as the events they depict-such as a live concert or distance learning session-occur) may be provided to end users.

[0062] Consumption rate prediction may observe the w_(obs) most recent rate samples to predict w_(pred) samples into the future. For example, if w_(obs)=10 and w_(pred)=2, the 10 previous rate samples may be used to predict the rate 2 samples into the future. The observation window R includes w_(obs) previous rate values <r₁, r₂, . . . , r_(obs)>, while prediction window {circumflex over (R)} includes the w_(pred) predicted rate values <{circumflex over (r)}₁, {circumflex over (r)}₂, . . . , {circumflex over (r)}_(wpred)>. The estimated future rate is denoted {circumflex over (r)}.

[0063] Prediction algorithms may be based on a number of different schemes. For example, an average consumption rate algorithm may be used, an exponential average algorithm may be used, or a fuzzy exponential average algorithm may be used.

[0064] An average consumption rate algorithm may predict the average consumption rate of the prediction window {circumflex over (R)} using an average consumption rate of the observation window R, according to Equation (6): $\begin{matrix} {\hat{r} = {\left( \frac{1}{w_{obs}} \right){\sum\limits_{i = 1}^{w_{obs}}r_{i}}}} & {{Equation}\quad (6)} \end{matrix}$

[0065] An exponential average consumption rate algorithm may be used to give more weight to some samples in the observation window than to others. A smoothed consumption rate parameter for i=1 is set to r₁, while the remainder of the SCR[i] are given by Equation (7) below, where α_(cr) is a weighting parameter.

SCR[i]=α _(cr) ×SCR[i−1]+(1−α_(cr))×r _(i−1)   Equation (7)

[0066] The estimated future rate is then given by Equation (8) below.

{circumflex over (r)}=SCR[w _(obs)+1]  Equation (8)

[0067] There are two variations in applying this algorithm to forecast the future consumption rates during the prediction window {circumflex over (R)}. The first variation, which will be referred to as the “expanding window exponential average algorithm,” predicts {circumflex over (r)}_(i) based on an increasing window <{circumflex over (R)}, {circumflex over (r)}₁, {circumflex over (r)}₂, . . . , {circumflex over (r)}_(i−1)> using Equation (8). The expanding window exponential average algorithm increases the window size by one sample each time a new {circumflex over (r)}_(i) is generated. The second variation, which will be referred to as the “sliding window exponential average algorithm,” keeps the window size constant and slides the observation window R forward when a new {circumflex over (r)}_(i) is generated.

[0068] A fuzzy exponential average consumption rate algorithm may be used to generate the {circumflex over (r)}_(i) by combining a fuzzy logic controller with the window exponential average algorithm. Using the fuzzy exponential average algorithm, the parameter α_(cr) is dynamically calculated.

[0069] The parameter α_(cr) controls the weight given to different samples. When α_(cr) is large, more weight is given to past samples. When α_(cr) is small, more weight is given to the more recent samples. Therefore, if the variability in the consumption rate in the system is small (i.e., the bit rate of the stream is fairly constant), the prediction error should be small, and a large α_(cr) may be used. On the other hand, if the variability is large (e.g., the stream is bursty), a small α_(cr) is appropriate, so that more recent sample data is weighted more heavily.

[0070] Referring to FIG. 5, a schematic 500 of the fuzzy exponential average algorithm is shown. Variability information 510 is provided to a fuzzy logic controller 520. Fuzzy logic controller 520 produces weighting factor α_(cr) 530 based on the variability information. The value for α_(cr) 530 and observation window information 540 is provided to the exponential average prediction algorithm 550, which outputs the prediction window information 560.

[0071] The variability of a stream may be characterized by a normalized variance var, calculated according to Equation (9) below. $\begin{matrix} {{var} = {\min \left( {1,\frac{{r_{obs} - \frac{\sum\limits_{i = 1}^{w_{obs}}r_{i}}{w_{obs}}}}{\frac{\sum\limits_{i = 1}^{w_{obs}}r_{i}}{w_{obs}}}} \right)}} & {{Equation}\quad (9)} \end{matrix}$

[0072] Referring to FIGS. 6A and 6B, membership functions for the variable var (FIG. 6A) and the variable α_(cr) (FIG. 6B) are shown. The following fuzzy control rules may be applied to the low, medium, and high regions of var data and α_(cr) data. If var is low, then α_(cr) is high. If var is high, then α_(cr) is low. If var is medium, then α_(cr) is medium. Of course, more complicated schemes may be used.

[0073] Feedback Message Delay

[0074] The round-trip feedback message delay (d_(feedback)) is an important factor in the transmission rate smoothing. The delay may be configured to be a conservatively estimated constant delay, or may be based on one or more measurements. The delay may be estimated dynamically, based on a prediction algorithm, to more closely reflect the transmission delay in the network.

[0075] The systems and techniques described herein can provide a number of benefits for streaming media data. Referring to FIG. 7, the normalized throughput standard deviation is shown for transmission of media data for the movie Twister, using a prediction window size of 90 seconds, three different buffer sizes (8, 16, and 32 MB), and for six different transmission methods. In the first (bar to the far right), complete rate information about the bit rate is known. In the second through fifth, MTFC was implemented with different numbers of intermediate thresholds. In the last, no smoothing was implemented.

[0076] As FIG. 7 demonstrates, using MTFC according to the current systems and techniques provides a significant benefit over unsmoothed transition of streaming data, with larger numbers of thresholds generally corresponding to smoother traffic. Additionally, FIG. 7 also demonstrates that providing a larger buffer provides for smoother traffic.

[0077]FIGS. 8A through 8C show the effect of varying the prediction window, for buffer sizes of 8 MB, 16 MB, and 32 MB, and for various numbers of intermediate thresholds. In FIGS. 8A-8C, larger prediction windows provide smoother traffic for larger buffer sizes, but not for smaller buffer sizes. This may be due to the fact that, for the same number of thresholds, the change in buffer level that triggers a change in server sending rate is much smaller (e.g., for a set number of thresholds, the “distance” between the thresholds in an 8 MB buffer is about {fraction (1/4)} of the distance between the thresholds in a 32 MB buffer). Therefore, with a larger buffer (with more distance between thresholds) there may be longer segments at a constant rate than with a smaller buffer having the same number of thresholds.

[0078] Feedback messages from the client to the server introduce overhead. In order to reduce consumption of network resources for control purposes, the overhead may be reduced by reducing the number of rate changes. However, there may be a trade-off between the number of rate changes and the smoothness of the traffic.

[0079] Referring to FIG. 9, an increase in the number of thresholds for a particular buffer size increases the number of rate changes (but also may provide for smoother traffic; see, e.g., FIG. 7). Additionally, for the same number of thresholds, larger buffers have fewer rate changes. Referring to FIGS. 10A-10C, a longer prediction window generally results in fewer rate changes.

[0080] Various implementations of the systems and techniques described here may be realized in digital electronic circuitry, integrated circuitry, specially designed ASICs (application specific integrated circuits), computer hardware, firmware, software, and/or combinations thereof. These various implementations may include implementation in one or more computer programs that are executable and/or interpretable on a programmable system including at least one programmable processor, which may be special or general purpose, configured to receive and/or transmit data and instructions, at least one input device, and at least one output device.

[0081] A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, different buffer sizes, threshold numbers, prediction window sizes, etc. may be used. Accordingly, other embodiments are within the scope of the following claims. 

What is claimed is:
 1. A method, comprising: receiving streaming media data from a server, the server transmitting the data at a first server transmission rate; storing at least some of the streaming media data in a buffer; determining a first buffer level of the buffer; subsequently determining a second buffer level of the buffer; comparing the first buffer level and the second buffer level to a plurality of buffer thresholds, the plurality of buffer thresholds including an underflow threshold, an overflow threshold, and a plurality of intermediate thresholds; determining whether at least one of the plurality of buffer thresholds is in a range from the first buffer level to the second buffer level; and if at least one of the plurality of buffer thresholds is in the range from the first buffer level to the second buffer level, determining a second server transmission rate based on the at least one buffer threshold.
 2. The method of claim 1, further comprising transmitting server transmission information based on the second server transmission rate to the server.
 3. The method of claim 2, wherein the server transmission information is chosen from the second server transmission rate and the difference between the first server transmission rate and the second server transmission rate.
 4. The method of claim 1, further comprising transmitting the second server transmission rate to the server if the second server transmission rate is different than the first server transmission rate.
 5. The method of claim 1, wherein determining a second server transmission rate based on the buffer threshold comprises determining a difference between the second buffer level and a target buffer level.
 6. The method of claim 1, wherein determining a second server transmission rate based on the buffer threshold comprises predicting one or more future consumption rates.
 7. The method of claim 1, wherein determining a second server transmission rate based on the buffer threshold comprises using a first calculation method to determine the second server transmission rate if the underflow threshold or the overflow threshold is not in the range from the first buffer level to the second buffer level.
 8. The method of claim 7, wherein determining a second server transmission rate based on the buffer threshold comprises using a second calculation method to determine the second server transmission rate if the underflow threshold or the overflow threshold is in the range from the first buffer level and the second buffer level.
 9. The method of claim 8, wherein determining a second server transmission rate based on the buffer threshold comprises predicting a consumption rate at a first time subsequent to a current time.
 10. The method of claim 9, wherein predicting a consumption rate at a first time subsequent to a current time comprises using a consumption rate at a second time previous to the current time to predict the consumption rate at the first time.
 11. The method of claim 10, wherein predicting a consumption rate at a first time subsequent to a current time comprises predicting the consumption rate using a prediction algorithm.
 12. The method of claim 11, wherein the prediction algorithm is chosen from the group consisting of an average consumption rate algorithm, an exponential average consumption rate algorithm, and a fuzzy exponential average algorithm.
 13. The method of claim 12, wherein the prediction algorithm is chosen from the group consisting of an exponential average consumption rate algorithm and a fuzzy exponential average algorithm, and wherein the prediction algorithm includes determining a smoothed consumption rate parameter based on one or more weighting factors.
 14. The method of claim 13, wherein the prediction algorithm is a fuzzy exponential average algorithm, and at least one of the one or more weighting factor is based on a variability of the streaming media data.
 15. A method, comprising: transmitting continuous media data to a client machine at a first transmission rate; receiving a communication from the client machine, the communication including rate change information determined by the client; and transmitting additional continuous media data to the client machine at a second transmission rate based on the rate change information, wherein the rate change information determined by the client is determined according to a method comprising: determining a first buffer level of a client buffer; subsequently determining a second buffer level of the client buffer; comparing the first buffer level and the second buffer level to a plurality of buffer thresholds, the plurality of buffer thresholds including an underflow threshold, an overflow threshold, and a plurality of intermediate thresholds; and determining whether at least one of the plurality of buffer thresholds is in a range from the first buffer level to the second buffer level, and if at least one of the plurality of buffer thresholds is in the range from the first buffer level to the second buffer level, determining a second server transmission rate based on the buffer threshold.
 16. An article comprising a machine-readable medium storing instructions operable to cause one or more machines to perform operations comprising: receiving streaming media data from a server, the server transmitting the data at a first server transmission rate; storing at least some of the streaming media data in a buffer; determining a first buffer level of the buffer; subsequently determining a second buffer level of the buffer; comparing the first buffer level and the second buffer level to a plurality of buffer thresholds, the plurality of buffer thresholds including an underflow threshold, an overflow threshold, and a plurality of intermediate thresholds; determining whether at least one of the plurality of buffer thresholds is in a range from the first buffer level to the second buffer level; and if at least one of the plurality of buffer thresholds is in the range from the first buffer level to the second buffer level, determining a second server transmission rate based on the buffer threshold.
 17. A system, comprising: means for receiving streaming media data from a server, the server transmitting the data at a first server transmission rate; means for storing at least some of the streaming media data in a buffer; means for determining a first buffer level of the buffer; means for subsequently determining a second buffer level of the buffer; means for comparing the first buffer level and the second buffer level to a plurality of buffer thresholds, the plurality of buffer thresholds including an underflow threshold, an overflow threshold, and a plurality of intermediate thresholds; means for determining whether at least one of the plurality of buffer thresholds is in a range from the first buffer level to the second buffer level; and means for determining a second server transmission rate based on a buffer threshold in the range from the first buffer level to the second buffer level.
 18. The system of claim 17, further comprising means for predicting a future consumption rate based on one or more prior consumption rates.
 19. The system of claim 17, further comprising means for communicating server transmission information to the server.
 20. The system of claim 17, further comprising means for storing the plurality of buffer thresholds. 